Specifies the subdirectory in which the kernel is located in on the boot partition. If not specified init will look in / and /boot , I'm not sure which order. As of puppy 3.00 this parameter also restricts the boot search for existing pup_save files only pup_saves inside this directory will used. This can be used to allow multiple puppy installation on the same partition to be used without risk of using the wrong one in error. example: psubdir=boot/puppy216

Not specifying this should automatically detect the boot media. Manually specify the boot media and interface using one of the following: usbflash usbhd usbcd ideflash idehd idecd idezip satahd satacd scsihd. This would be where USB, IDE, SATA, SCSI are the accepted interfaces and flash memory, hard drive, CD/DVD drive, ZIP drive are the accepted drives, e.g. PMEDIA=idehd.